Add help text to the DynamicFormType

[FlorianBoe]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	yes
Deprecations?	no
Fixed tickets
Related issues/PRs	#344
License	MIT

What's in this PR?

I have added the help property to the DynamicFormType. The field is displayed below the rest of the form fields. The field type is text_editor and the information is set to the help property of Symfony forms, with the help_html option enabled.

I tried to implement the property analog to placeholders. Feel free to look over it again thoroughly, I'll be honest, I haven't looked closely at the details of the whole implementation.

Why?

Editors can now add additional help text to form fields.

To Do

• Tests
• Create documentation

[FlorianBoe]

To be honest, I'm not quite sure how to fix the PHP Lint error. Why is there a problem with null on the new properties on getValue that the other properties don't have?

I would be grateful for support!

[alexander-schranz]

@FlorianBoe you can ignore at current state the phpstan issues for this PR :) Did not yet have deep look at it. But we require a line in the UPGRADE.md about SQL migration for the new field.